1. Field of the Invention
The present invention relates to a method and apparatus for efficiently coding a continuous tone image.
2. Description of the Prior Art
Bit plane coding is a conventional coding scheme of the type described above. According to this scheme, an n-bit signal represents 2.sup.n gray scale levels. An Mth (M=1 to n) bit signal of each picture element is coded as one imaginary image when each gray scale level is represented by a binary number.
Another conventional coding scheme is block truncation coding. According to this coding scheme, an image is divided into blocks each consisting of K.sub.1 .times.K.sub.2 picture elements. An average value P.sub.0 of the signal levels is calculated for each block. The signal level of a given picture element, which exceeds the average value P.sub.0, is replaced with an average value P.sub.1 of such picture elements. The signal level of another given picture element, which is below the average value P.sub.0, is replaced with an average value P.sub.2 of such picture elements. The picture element information is then approximated and coded on the basis of the two levels represented by the average values P.sub.1 and P.sub.2, and its position data.
In 2.sup.n gray scale representation in the conventional bit plane coding scheme, n imaginary images constituted by each bit are coded as a black or white image represented by "1" or "0". This scheme can be realized by using a plurality of black-and-white image coding circuits. It is difficult to achieve coding with this method with high efficiency.
Since each block information is represented by two levels in the conventional block truncation coding scheme, coding with high efficiency can be achieved. However, the gray scale levels are limited to two levels, and requirements for high image quality cannot be satisfied.